In the manufacture of printed circuitry it is customary to provide a surface of a silicon wafer with a photoresist film which is then illuminated through a suitable mask in order to form a pattern of exposed and unexposed areas on an underlying oxide layer, either the exposed or the unexposed portions being thereafter chemically removed to enable a selective etching of the oxide layer. In many instances the same pattern is repetitively projected onto different parts of tthe wafer for the mass production of identical chips. A precise alignment of the wafer parts to be illuminated with the mask carried by an overlying holder is essential; for this purpose the apparatus may include a prepositioning station in which its support, usually designed as a rotatable chuck, is displaceable with the aid of certain markings on the wafer for proper orientation before that wafer is transferred to an exposure station. In the system described in my above-identified copending application, such prealignment is achieved by training several beams of parallel light rays onto a noncircular segment of the wafer edge and measuring the intensity of the beam portions not intercepted by the wafer to determine the proper angular position; some of the rays in the intercepted beam portions fall upon marks spaced inward from the wafer periphery whose reflections are used for a more precise alignment.
When the properly oriented wafer is subsequently moved from the prepositioning station to another stage in the exposure station, its temperature should substantially match that of the exposure stage, because dimensional changes due to variations of the wafer temperature may distort the pattern projected upon the photoresist film through the photomask. Conventional wisdom is to maintain a substantially uniform temperature throughout the apparatus, including the exposure stage, the mask holder and the intervening structure, as by the circulation of heating or cooling air. If, however, the temperature equilibrium of such a system is disturbed it takes considerable time to re-establish it.
On the other hand it has been proposed by Ernst W. Lobach (VLSI Wafer Printing with Step- and Repeat Aligners, presented at IGC Conference, Amsterdam, Sept. 19-21 1979) to control the temperature of the wafer independently thereby offsetting variations in the temperature of other components. No indication has been given, however, what means could be used to increase and particularly to decrease the temperature of the wafer within a short interval.